The dynamics of large-scale winds in nearby starburst galaxies

Abstract

We present detailed spectroscopic and multiband photometric observations of the nearby galaxy M82 in order to study the high-velocity outflows observed in such galaxies as a property of the energetic starburst phenomena associated with their nuclei. The high spatial and kinematic resolution of our observations has allowed us to perform photometric analysis of H$\alpha$, (N II), and (O III) spectral lines at roughly one hundred thousand positions across the extent of the galaxy. The observed velocities of the H$\alpha$-emitting gas in M82 suggest a bipolar outflow of material along the minor axis at a projected velocity of $\sim$300 km/s, fueled by the bright nuclear starburst regions in the galaxy's disk. All three spectral lines show double components in the centers of the outflowing lobes, with the H$\alpha$ line split by $\sim$300 km/s over a region almost a kiloparsec in size. We argue for a model in which the optical emission is radiated by denser ambient material on the surface of "bubbles" that have been evacuated by a hot wind ($\sim$10\sp8 K) visible at x-ray wavelengths. The outflow is confined to a cylinder within 350 pc of the disk, but flares outward in a cone beyond that point. The optical line-emitting filaments consist of both gas that has been entrained from the disk by the outflow and material already present in the halo of M82. Although the detailed structure of the bubbles is complex, we confirm the major predictions of galactic wind hydrodynamical simulations. Line ratio maps reveal high (N II) /H$\alpha$ in the disk, suggesting the presence of a diffuse ionized medium (DIM) similar to that seen in NGC 891 and other star forming galaxies. The halo of M82 is also observed in emission lines, but only as dust-scattered disk radiation. We conclude that M82 has an active star-forming disk, a dusty scattering halo, and a bipolar starburst-driven wind